1. Field of the Invention
The present invention relates generally to purge or bubble tube devices used for the measurement of the depth of a quantity of liquid. More specifically, the present invention relates to a liquid depth sensing and identification system that incorporates a novel differential pressure sensor and/or a novel mass flow sensor to detect the bubble signature.
2. Description of the Related Art
The purge or bubble tube principle of detecting the depth of a quantity of liquid in a tank or container is well known. The present inventor has found more recently that it is possible to detect certain characteristics in the propagation of the bubbles as they leave the purge tube, or more precisely, certain characteristics in the variation of the pressure pulses as the bubbles are emitted from the end of the tube. These characteristics have been found to correlate with the type of liquid found at the end of the purge tube. Such a system is described in the present inventor's issued U.S. Pat. No. 6,931,926.
The liquid depth sensing and identification system of the present inventor's issued '926 U.S. Patent has been found to work quite well in the identification of various liquid types and can be used to detect contaminants, etc., in the bottom of a tank of liquid, e.g., water in a fuel tank, etc. The system described in the above noted '926 U.S. Patent relies upon the measurement of the pressure pulses over a period of time as the bubbles leave the end of the purge tube. The pressure variations are converted to a voltage, with the voltage generating a display of the pressure differences on a chart recorder or the like to indicate the specific “bubble signature” of the type of liquid at the end of the purge tube.
A problem with such a purge-type liquid identification system is that the pressure variations that create the “bubble signatures,” and thus enable the system to determine the type of liquid at the end of the purge tube, are quite small. In contrast, the absolute pressure developed at the lower end of the purge tube due to the depth of the liquid in the tank or container can be quite high. This is particularly true in the case of a tank or container of relatively great depth, e.g., an underground gasoline storage tank of several feet in height, etc.
Looking at this problem in another way, it might be considered analogous to a need to magnify a very small portion of a large object in order to detect minute distinctions in the object. When a microscope is brought to bear upon the area in question, the minute distinctions may be readily visible, but the overall size or quantity of the object is not. When the magnification is reduced in order to bring the entire object into the field of view, the resolution needed is no longer available to detect the minute variations in the portion of interest in the object.
Returning to the principles of the present invention, it can be extremely difficult to detect the very small distinctions in the bubble pressure pulse variations due to different types of liquids while still providing the range necessary to measure the pressure head of the liquid in the tank between its full and empty depths. Thus a liquid depth sensing and identification system solving the aforementioned problems is desired.